The amount and type of ventilation depends on your climate. The best ventilation is protected from the weather, predator proof, and high off the floor (warm, moist air rises). Multiple vents placed opposite from each other provides cross ventilation to pull out the warm moist air from droppings and the chickens' breath. In areas with cold winters aim for at least one square foot of vents per bird. You can't go wrong with open soffits covered in 1/2" hardware cloth high above the roosts, plus windows (also covered in hardware cloth) that open for warm days. In areas that stay warm year round open air coops (mostly hardware cloth walls) are great for keeping the chickens cool and dry.